Abstract

The primary occurrence of ruby in the Mogok area, northern Myanmar is exclusively found in marble along
with spinel–forsterite-bearing marble and phlogopite–graphite marble. These marble units are enclosed
within banded biotite–garnet–sillimanite–oligoclase gneisses. Samples of these marbles collected for C–O
stable isotope analysis show two trends of δ13C–δ18O variation resulting most likely from fluid–rock
interactions. Ruby-bearing marble and phlogopite–graphite marble followa trend with coupled C–O depletion,
whereas spinel–forsterite-bearing marble follows a δ18O depletion trend with relatively constant δ13C values.
Ruby formation might have resulted from CO2-rich fluid–rock interaction, while spinel–forsterite-bearing
marble was genetically related to CO2-poor fluid–rock interaction. Both fluids may have arisen from external
sources. Based on graphite Raman spectral thermometry, the estimated temperature for phlogopite–graphite
marble, and probably ruby-bearing marble, was lower than 607 °C, and for spinel–forsterite-bearing marble,
lower than 710 °C. Contrasting C/O diffusion between graphite/ruby/spinel/forsterite and calcite, local
variations of isotopic compositions of newly formed minerals as a result of non-pervasive fluid infiltration, and open-system isotopic disturbance during cooling may have affected C-/O-isotopic fractionations between
minerals. The estimated high formation temperatures for ruby and spinel/forsterite imply that the parental
fluidsmay have been related to nearby igneous intrusions and/or metamorphic processes. Whether these two
types of fluid were genetically related is unclear based on the present data.